Steel for rails and a method of manufacturing of a rail thereof

ABSTRACT

A steel for rail having the following elements, 0.25%≤C≤0.8%; 1.0%≤Mn≤2.0%; 1.40%≤Si≤2%; 0.01%≤Al≤1%; 0.8%≤Cr≤2%; 0≤P≤0.09%; 0≤S≤0.09%; 0%≤N≤0.09%; 0%≤Ni≤1%; 0%≤Mo≤0.5%; 0%≤V≤0.2%; 0%≤Nb ≤0.1%; 0%≤Ti≤0.1%; 0%≤Cu≤0.5%; 0%≤B≤0.008%; 0%≤Sn≤0.1%; 0% ≤Ce≤0.1%; 0%≤Mg≤0.10%; 0%≤Zr≤0.10%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure having, by area percentage, 2% to 10% of Proeutectoid Ferrite, the balance being made of Pearlite wherein the pearlite having interlamellar spacing from 100 nm to 250 nm .

The present invention relates to a steel suitable for manufacturing ofrails for railways and particularly for trains running on magneticlevitation or magnetic guiding based on repulsion and attractionprinciples.

BACKGROUND

Steels for the rails are developed for high speed railway or for dualuse that is for both freight and passenger railways. Irrespective of theuse the load carrying capacity of the railways has increased and it isexpected to increase in future.

SUMMARY OF THE INVENTION

Hence it is necessary to develop steels for rails are good inmechanical, electric and magnetic properties such as resistivity,permeability and tensile strength even in the harsh working environmentfor the rails.

Therefore, intense research and development endeavors are beingundertaken put in to develop a material that is good in resistivity andpermeability while having high tensile strength at room temperature aswell as at a temperature of 180° C. that is above 900 MPa with adequatehardness.

Earlier research and developments in the field of steels for rails forrailways have resulted in several methods for producing high strengthand wear resistant steel for rails some of which are enumerated hereinfor conclusive appreciation of the present invention:

US4350525 magnetic suspension railroad magnetically active part is madeof steel with the composition-0 to 0.15—% carbon, 0 to 0.045—%phosphorus, 0 to 0.008—% nitrogen, 0.75 to 2.0—% silicon, 0.15 to 1.00—%manganese, 0.02 to 0.07—% aluminum, soluble, 0.25 to 0.55—% copper, 0.65to 1.00—% chromium, remainder—iron with unavoidable impurities but thesteel of US4350525 does not demonstrate to reach the tensile strength of900 MPa at 180° C.

WO2016019730 is a F-shaped rail for the induction core made of softmagnetic steel, and the chemical composition of the soft magnetic steelis C: 0.005% to 0.15% by weight , Mn: 0.25% to 0.60%, Si: 0.30% to 1.0%,Re: 0.003% to 0.006%, P and S are both less than 0.025%, the rest is Feand trace impurities but this steel is also not able to reach thestrength of 900 MPa at a temperature of 180° C.

An object of the present invention is to make available a steel suitablefor mechanical operations for manufacturing rails for railways thatsimultaneously have:

-   -   a tensile strength greater than or equal to 900 MPa and        preferably above 920 MPa at 180° C.,    -   a hardness of at least 310 Hv or more and preferably more than        315 Hv or more    -   Resistivity of 40 Ωmm/m² or more and preferably 41 Ωmm/m² or        more    -   A maximum permeability of 165 or more measured at 4000 A/m.

In a preferred embodiment, the steel according to the invention may alsohave a tensile strength greater than or equal to 950 MPa and preferablyabove 1000 MPa at room temperature,

In a preferred embodiment, the steel according to the invention may alsohave a polarization of more than 1.5 T measures at 40000 A/m.

In a preferred embodiment, the steel according to the invention may alsohave a Flux Density of more than 1.5 T measures at 40000 A/m.

Preferably, such steel is suitable for manufacturing of rails and thesteel is also suitable for other structural parts of rails such aschassis members of the rail wagon.

Another object of the present invention is also to make available amethod for the manufacturing of these mechanical parts that iscompatible with conventional industrial applications while being robusttowards manufacturing parameters shifts.

Carbon is present in the steel of the present invention from 0.25% to0.8%. Carbon is an element necessary for increasing the strength of theSteel of the present invention by producing pearlite. Carbon alsoensures the resistivity by assisting in the formation of cementite inlamellar pearlite. But Carbon content less than 0.25% will not be ableto impart the tensile strength as well as resistivity due to theexcessive formation of Proeutectoid ferrite. On the other hand, at aCarbon content exceeding 0.7%, the tensile strength is adverselyimpacted due to the excessive formation of proeutectoid cementite duringthe cooling after hot rolling. Further excessive formation ofproeutectoid cementite is also detrimental for the rail during itsoperational life cycle. The carbon content is advantageously in therange 0.27% to 0.75% and more especially 0.28% to 0.7%.

Manganese is added in the present steel from 1.0% to 2.0%. Manganeseprovides solid solution strengthening, and increases the hardenabilityby assisting in the formation of cementite in pearlite therebyincreasing the resistivity. Further Manganese also suppresses theferritic transformation temperature and reduces ferritic transformationrate to control the formation of Proeutectoid ferrite hence assisting inthe formation of pearlite. An amount of at least 1.0% is required toimpart strength as well as to assist the formation of Pearlite. But whenManganese content is more than 2.0% it produces adverse effects such asit speed-up the transformation of Austenite to Martensite or bainiteduring cooling after hot rolling which are detrimental for the steel ofthe present invention as these microstructure adversely affects theresistivity and permeability of the steel of the present invention.Manganese content of above 2.0% can also get excessively segregated inthe steel during solidification and homogeneity inside the material isimpaired which can cause surface cracks during a hot working process.The preferred limit for the presence of Manganese is from 1.0% to 1.8%and more preferably from 1.0% to 1.5%.

Silicon is an essential element that is present in the steel of thepresent invention from 1.40% to 2%. Silicon imparts the steel of thepresent invention with strength through solid solution strengthening andalso acts as a deoxidizer. But as Silicon is a ferrite former it alsoincreases the Ac3 transformation point which will push the austenitictemperature to higher temperature ranges. That is why the content ofSilicon is kept at a maximum of 2%. Silicon content above 2% can alsocause temper embrittlement.The preferred limit for the presence ofSilicon is from 1.45% to 1.8% and more preferably from 1.45% to 1.6%.

The content of the Aluminum is from 0.01% to 1%. Aluminum removes Oxygenexisting in molten steel to prevent Oxygen from forming a gas phaseduring solidification process. Aluminum also fixes Nitrogen in the steelto form Aluminum nitride to reduce the size of the grains. Aluminumallows the steel of the present invention to have control over the sizeof the pearlite lamellar spacing and thereby increase the resistivitywhile retaining adequate permeability. Higher content of Aluminum above1% leads to the occurrence of coarse aluminum-rich oxides thatdeteriorate fatigue limit and brittle fracture of steel rail. Thepreferred limit for the presence of Aluminium is from 0.02% to 0.9% andmore preferably from 0.02 to 0.5%

Chromium is present from 0.8% to 2% in the steel of the presentinvention. Chromium is an essential element that provide strength to thesteel by solid solution strengthening and a minimum of 0.2% is requiredto impart the strength but when used above 2% increase the hardenabilitybeyond an acceptable limit due the formation of undesired phases such asbainite after cooling thereby impairing the ductility of the steel.Chromium addition above 2% also decreases the diffusion coefficient ofcarbon in the austenite and hence retards the formation pearlite duringthe cooling after hot rolling. The preferred limit for the presence ofChromium is from 0.9% to 1.9% and more preferably from 0.9% to 1.6%.

Phosphorus is content of the steel of the present invention is from 0%to 0.09%.

Phosphorus tends to segregate at the grain boundaries or co-segregatewith Manganese. For these reasons, it is recommended to use phosphorusas less as possible. Specifically, content over 0.09% can cause ruptureby intergranular interface decohesion which may be detrimental for thetensile strength and wear resistance. The preferred limit for Phosphoruscontent is from 0% to 0.05%.

Sulphur is contained from 0% to 0.09%. Sulphur forms MnS precipitateswhich can become elongated. Such elongated MnS inclusions can haveconsiderable adverse effects on mechanical properties such as hardnessand tensile strength if the inclusions are not aligned with the loadingdirection. Therefore sulfur content is limited to 0.09%. A preferablerange the content of Sulphur is 0% to 0.05% and more preferably 0% to0.02%.

Nitrogen is in an amount from 0% to 0.09% in steel of the presentinvention. Nitrogen is limited to 0.09% to avoid ageing of material andprevent the precipitation of coarse Aluminum nitrides duringsolidification which are detrimental for mechanical properties of thesteel. Nitrogen also forms nitrides and carbonitrides with vanadiumtitanium and niobium to impart strength to the steel of the presentinvention.

Nickel is an optional element and added to the present invention from 0%to 1% to increase the strength of the steel of the present invention.Nickel is beneficial in improving its pitting corrosion resistance.Nickel is added into the steel composition to decrease the diffusioncoefficient of carbon in the austenite thereby promoting the formationof Ferrite in pearlite. But the presence of nickel content above 1% maylead to the stabilization of residual austenite thereby having adetrimental impact on tensile strength. It is preferred to have nickelfrom 0% to 0.9% in the steel of the present invention.

Molybdenum is an optional element and may be present from 0% to 0.5% inthe present invention. Molybdenum is added to impart hardenability andhardness to steel by forming Molybdenum based carbides. However, theaddition of Molybdenum excessively increases the cost of the addition ofalloy elements, so that for economic reasons its content is limited to0.5%. The preferred limit for molybdenum content is from 0% to 0.4% andmore preferably from 0% to 0.2%.

Vanadium is an optional element for the present invention and is contentis from 0% to 0.2%. Vanadium is effective in enhancing the strength ofsteel by precipitation strengthening especially by forming carbides orcarbo-nitrides. The upper limit is kept at 0.2% due to the economicreasons.

Niobium is present in the Steel of the present invention from 0% to 0.1%and suitable for forming carbo-nitrides to impart strength of the Steelof the present invention by precipitation hardening. Niobium will alsoimpact the size of microstructural components through its precipitationas carbo-nitrides and by retarding the recrystallization during heatingprocess and thus refining the grain size. However, Niobium content above0.1% is not economically interesting as well as forms coarserprecipitates which are detrimental for the tensile strength of the steeland also when the content of niobium is 0.1% or more niobium is alsodetrimental for steel hot ductility resulting in difficulties duringsteel casting and rolling.

Titanium is an optional element and present from 0% to 0.1%. Titaniumforms titanium nitrides which impart steel with strength and refine thegrain size. The preferred limit for titanium is from 0% to 0.05%.

Copper is a residual element and may be present up to 0.5% due toprocessing of steel. Till 0.5% copper does not impact any of theproperties of steel but over 0.5% the hot workability decreasessignificantly.

Other elements such as Tin, Cerium, Magnesium, boron or Zirconium can beadded individually or in combination in the following proportions byweight: Tin≤0.1%, Cerium≤0.1%, Magnesium≤0.10%, 0%≤Boron≤0.008% andZirconium≤0.10%. Up to the maximum content levels indicated, theseelements make it possible to refine the grain during solidification. Theremainder of the composition of the Steel consists of iron andinevitable impurities resulting from processing.

The microstructure of the Steel comprises:

Pearlite is the matrix microstructural constituent of the present steeland the area percentage presence must be at least 90% or more and it ispreferred from 90% to 99% and more preferably from 93% to 98%. Pearliteis formed during the second step of cooling after hot rolling. Pearliteof the present steel is of lamellar structure. The lamellar structure ofthe pearlite of present invention is an aggregate of ferrite andcementite and the inter-lamellar spacing of the pearlite of presentinvention is from 100 nanometers to 250 nanometers. This inter-lamellarspacing improves the in-use properties of the steel of the presentinvention such as tensile strength, and resistivity. When theinter-lamellar spacing is more than 250 nanometer the steel will be softand is not able to reach the tensile strength an especially the tensilestrength at 180° C. and whenever the inter-lamellar spacing of thepearlite is less than 100 nanometers the permability of the steel isadversely affected. The preferred limit for the inter-lamellar spacingis from 110 nanometers to 230 nanometers and more preferably from 120nanometers to 220 nanometers. Pearlite of the present invention alsoimpart the steel with in-use properties like Permeability and hardness.

Proeutectoid ferrite is present from 2% to 10% in the steel of thepresent invention. Proeutectoid ferrite is formed during the first stepof cooling after hot rolling on the grain boundaries of the prioraustenite grains and Proeutectoid ferrite interspersed within thepearlite. Proeutectoid ferrite provides the present steel with ductilityas well as the permeability. If the content of the Proeutectoid ferriteis more than 10% then the steel of the present invention will not beable to achieve hardness. Preferred limit for the presence ofProeutectoid ferrite is from 3% to 9% and more preferably from 3% to 8%.

In addition to the above-mentioned microstructure, the microstructure ofthe rail is free from microstructural components such as bainite,martensite and residual austenite.

A rail according to the invention can be produced by any suitablemanufacturing process, with the stipulated process parameters explainedhereinafter.

A preferred exemplary method is demonstrated herein but this exampledoes not limit the scope of the disclosure and the aspects upon whichthe examples are based. Additionally, any examples set forth in thisspecification are not intended to be limiting and merely set forth someof the many possible ways in which the various aspects of the presentdisclosure may be put into practice.

A preferred method consists in providing a semi-finished casting ofsteel with a chemical composition according to the invention. Thecasting can be done in any form such as ingots or blooms or billetswhich is capable of being manufactured or processed into a rail forrailways and particularly for magnetic levitation rails.

For example, the steel having the above-described chemical compositionis casted in to a billet and then rolled in the form of a bar. This barcan act as a semi-finished product for further rolling. Multiple rollingsteps may be performed to obtain the desired semi-finished product.

In order to prepare for the steel to be manufactured as a rail, thesemi-finished product can be used directly at a high temperature afterrolling or may be first cooled to room temperature and then reheated formanufacturing the rail.

The semi-finished product is reheated from temperature Ac3 to Ac3+500°C., preferably from Ac3+30° C. to Ac3+450° C. and more preferably from1100° C. to 1300° C. where it is held during 5 seconds to 1200 secondsto ensure homogenous temperature across the cross section of thesemi-finished product as well as to ensure 100% austenite is formed. TheAc3 is calculated according to KASATKIN, O.G. et al. Calculation Modelsfor Determining the Critical Points of Steel in Metal Science and HeatTreatment, 26:1-2, January-February 1984, 27-31.

If the reheating temperature of the semi-finished product is lower thanAc3, excessive load is imposed during the rolling further, thetemperature of the steel may also decrease below the Ferritetransformation start temperature that will lead to the ferrite formationin during the hot rolling. Additionally the metallurgical transformationunder strain can lead to significant change in the obtainedmicrostructure for a given cooling rate or a given chemical composition.As a result, the obtained microstructure will be completely differentfrom the targeted one and so the mechanical properties as well as theelectrical properties. Therefore, the temperature of the semi-finishedproduct is preferably sufficiently high so that all the mechanicaloperations are performed and completed in the 100% austenitictemperature range. Reheating at temperatures above Ac3+500° C. must beavoided because they are industrially expensive and can lead to theoccurrence of liquid areas that will affect the rolling of the steel.

Then the semi-finished is subjected to at least one pass of hot rollingfrom Ac3 to Ac3+300° C., preferably with a reduction from 35 to 90%. Hotrolling may be done in multiple passes that are required to have a hotrail from semi finished product. The preferred temperature for all thehot rolling is from Ac3+30° C. to Ac3+300° C. and more preferabletemperature is from Ac3+50° C. to Ac3+250° C.

A final rolling temperature must be kept above Ac3 and this is preferreda structure that is favorable to recrystallization and mechanicalmanufacturing. It is preferable to have all the rolling pass especiallythe final rolling temperature to be performed at a temperature greaterthan 1000° C., because below this temperature the steel exhibits asignificant drop in the rollability. In case the final rollingtemperature is less than Ac3 it can lead to issues regarding the finaldimension of the rail as well as a deterioration of the surface aspect.It can even provoke cracks or a full failure of the rail.

The hot rail is then cooled in a two step cooling process wherein thefirst step of cooling starts from the exit of final hot rolling, the hotrail being cooled down, at a cooling rate CR1 from 0.1° C./s to 5° C./s,to a temperature T1 which is in a range from 480° C. to 550° C. In apreferred embodiment, the cooling rate CR1 for such first step ofcooling is from 0.1° C./s to 3° C./s and more preferably from 0.1° C./sto 2° C./s. The preferred T1 temperature for such first step is from490° C. to 530° C. and more preferably from 490° C. to 510° C.

In the second step of cooling, the hot rail is cooled down from T1 toroom temperature, at a cooling rate CR2 which is less than 5° C./s. In apreferred embodiment, the cooling rate CR2 for the second step ofcooling is less than 3° C./s and more preferably is less than 1° C./s.

In a preferred embodiment, CR1 is higher than CR2.

When the hot rail reaches room temperature the rail is obtained from thesteel of the present invention.

EXAMPLES

The following tests, examples, figurative exemplification and tableswhich are presented herein are non-restricting in nature and must beconsidered for purposes of illustration only and will display theadvantageous features of the present invention.

Rails made of steels with different compositions is gathered in Table 1,where the rail is produced according to process parameters as stipulatedin Table 2, respectively. Thereafter Table 3 gathers the microstructuresof the rail obtained during the trials and table 4 gathers the result ofevaluations of obtained properties.

TABLE 1 Steel Samples C Mn Cr Si Al S P N Mo Cu Ni V Ti I1 0.50 1.081.01 1.53 0.47 0.0010 0.007 0.004 0.0058 0.0130 0.006 0.008 0.0012 I20.51 1.08 1.04 1.54 0.06 0.0010 0.007 0.004 0.0058 0.011 0.006 0.00050.0011 I3 0.69 1.29 1.27 1.52 0.03 0.0011 0.004 0.004 0.0070 0.013 0.0060.0008 0.0013

TABLE 2 HR Reduc- Steel Reheating Finish tion CR1 T1 CR2 Ac3 Sample (°C.) (° C.) % (° C./s) (° C.) (° C./s) (° C.) I1 1250 1035 67 0.6 500 0.1848 I2 1250 1035 67 0.6 500 0.1 817 I3 1250 1035 67 0.6 500 0.1 800

Table 2 gathers the process parameters implemented on semi-finishedproduct made of steels of Table 1. The trials 11 to 13 serve for themanufacture of rail according to the invention. The table 2 is asfollows:

Ac3 values were determined through KASATKIN, O.G. et al. CalculationModels for Determining the Critical Points of Steel in Metal Science andHeat Treatment, 26:1-2, January-February 1984, 27-31.

TABLE 3 Pearlite Steel Pearlite Proeutectoid interlamellar Sample %ferrite % spacing (nm) I1 95 5 125 I2 95 5 170 I3 97 3 211

Table 3 exemplifies the results of the tests conducted in accordancewith the standards on different microscopes such as Scanning ElectronMicroscope for determining the microstructures of both the inventive andreference steels in terms of area fraction. The results are stipulatedherein:

TABLE 4 Max TS at Room TS at Permeability Sample Temperature 180° C.Resistivity measured at Steels (MPa) (Mpa) HV20 (Ωmm2/m) 4000 A/m I11031 937 319 45.8 184 I2 1018 921 312 41.4 186 I3 1193 1049 350 42.3 170

Table 4 exemplifies the mechanical properties and magnetic properties ofboth the inventive steel and reference steels. In order to determine thetensile strength, tests are conducted in accordance of NF EN ISO6892-1/2017 standards. Tests to measure the resistivity and permeabilityfor both inventive steel and reference steel are conducted in accordanceof IEC-60404-13 and IEC-60404-4 respectively . Tests to measure thehardness for both inventive steel and reference steel are conducted inaccordance of EN-13674. The results of the various mechanical testsconducted in accordance to the standards are gathered.

What is claimed is: 1-15. (canceled)
 16. A steel for rail comprising ofthe following elements, expressed in percentage by weight: 0.25%≤C≤0.8%;1.0%≤Mn≤2.0%; 1.40%≤Si≤2%; 0.01%≤Al≤1%; 0.8%≤Cr≤2%; 0≤P≤0.09%;0≤S≤0.09%; 0%≤N≤0.09%; and optionally including one or more of thefollowiing elements 0%≤Ni≤1%; 0%≤Mo≤0.5%; 0%≤V≤0.2% 0%≤Nb≤0.1%;0%≤Ti≤0.1%; 0%≤Ce≤0.1%; 0%≤Mg≤0.10%; 0%≤Zr≤0.10%; a remaindercomposition being composed of iron and unavoidable impurities caused byprocessing, a microstructure of the steel comprising, by areapercentage, 2% to 10% of Proeutectoid Ferrite, the balance being made ofPearlite wherein the pearlite has interlamellar spacing from 100 nm to250 nm.
 17. The steel as recited in claim 16 wherein the compositionincludes 0.27% to 0.75% of Carbon.
 18. The steel as recited in claim 16wherein the composition the composition includes 0.02% to 0.9% ofAluminum.
 19. The steel as recited in claim 16 wherein the compositionthe composition includes 0.9% to 1.9% of Chromium.
 20. The steel asrecited in claim 16 wherein the Pearlite is between 93% and 99%.
 21. Thesteel as recited in claim 16 wherein the interlamellar spacing ofpearlite is from 110 nm to 230 nm.
 22. The steel as recited in claim 16wherein a tensile strength at 180° C. is greater than 900 MPa.
 23. Thesteel as recited in claim 16 wherein the steel has hardness of 310 Hv ormore.
 24. The steel as recited in claim 16 wherein the steel has aresistivity of more than 40 Ωmm/m².
 25. The steel as recited in claim 16wherein the steel has a maximum permeability equal to more than 165 ormore measure at 4000 A/m.
 26. A method of production of a rail of steel,the method comprising the following successive steps: providing a steelcomposition in the form of semi-finished product, the steel compositionexpressed in percentage by weight including: 0.25%≤C≤0.8%; 1.0%≤Mn≤2.0%;1.40%≤Si≤2%; 0.01%≤Al≤1%; 0.8%≤Cr≤2%; 0≤P≤0.09%; 0≤S≤0.09%; 0%≤N≤0.09%;and optionally including one or more of the followiing elements0%≤Ni≤1%; 0%≤Mo≤0.5%; 0%≤V≤0.2% 0%≤Nb≤0.1%; 0%≤Ti≤0.1%; 0%≤Cu≤0.5%;0%≤B≤0.008%; 0%≤Sn≤0.1%; 0%≤Ce≤0.1%; 0%≤Mg≤0.10%; 0%≤Zr≤0.10%; aremainder composition being composed of iron and unavoidable impuritiescaused by processing; reheating the semi-finished product to atemperature from Ac3 to Ac3+500° C. and holding at the temperature from5 seconds to 1200 seconds; performing one or more hot rolling passes onthe semi-finished product in the austenitic range at a hot rollingtemperature from Ac3 to Ac3+300° C. to obtain a hot rail; and coolinghot rail in two-step cooling, wherein in step one the hot rail is cooledat a cooling rate from 0.1° C./s to 5° C./s from Ac3 and Ac3+300° C.temperature to a temperature T1 ranging from 480 to 550° C., andthereafter in step two the hot rail is cooled at a cooling rate less 5°C./s from T1 to room temperature to obtain a rail.
 27. The method asrecited in claim 26 wherein the reheating temperature of thesemi-finished product to is from Ac3+30° C. to Ac3+450° C.
 28. Themethod as recited in claim 26 wherein the temperature T1 is from 490° C.to 530° C.
 29. The method as recited in claim 26 wherein the CR1 coolingrate is higher than CR2.
 30. A method for the manufacture of structuralor safety parts of a rail wagon comprising the method as recited inclaim
 26. 30. A method for the manufacture of structural or safety partsof a rail wagon comprising the employing the steel as recited in claim16.